Identify the mechanisms by which bone metabolism is altered in breast cancer. To do this, we will: determine the cellular mechanism of bone destruction in metastatic breast cancer; determine the humoral factors responsible for hypercalcemia and/or bone metastases in breast cancer; characterize the effects of these factors on bone and identify ways in which these effects can be prevented or opposed; investigate pharmacologic inhibition of the bone resorbing vactors produced by breast cancer cells; and investigate the production of substances which stimulate new bone formation and collagen synthesis in breast cancer.